Tracking Issue for `iter` macro

This is a tracking issue for the iter macro lang team experiment.
The feature gate for the issue is #![feature(iter_macro)].

Links to the discussion are in this #t-lang/gen topic, with more context starting here: https://github.com/rust-lang/rust/pull/137725#issuecomment-2702729792.

See also:

• https://github.com/rust-lang/rust/issues/117078

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label. Discussion comments will get marked as off-topic or deleted. Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the macro
• Experiment with it in nightly
• Write an RFC: https://github.com/rust-lang/rfcs/pull/3861

Unresolved Questions

• Which syntaxes we support:
  • iter!(|| { ... })
  • iter! { ... }
• Which traits the return value of each syntax implements:
  • Iterator
  • IntoIterator
  • Fn() -> impl Iterator
• https://github.com/rust-lang/rust/issues/153688

Implementation history

• #137725

Start with RFC #3861, implementation PR #137725, and the related issue #153688, then review the unresolved syntax and return-trait questions listed here. Done means the design questions are resolved and the tracking checklist reflects the path toward stabilizing the iter macro.